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A tire pressure monitoring system (TPMS) is an electronic system designed to monitor the air pressure inside the pneumatic tires on various types of vehicles. The system is also sometimes referred to as a tire-pressure indication system (TPIS). These systems report real-time tire-pressure information to the driver of the vehicle, either via a gauge, a pictogram display, or a simple low-pressure warning light. TPMS systems can be divided into two different types; "direct" and "indirect". Direct systems can be further classified into battery powered and battery-less systems. TPMS are provided both at an OEM (factory) level as well as aftermarket solution.

Due to the significant influence tire pressure has on vehicle safety and efficiency, TPMS was first adopted widely by the European market as an optional feature for luxury passenger vehicles in the 1980s. The first passenger vehicle to adopt tire-pressure monitoring (TPM) was the Porsche 959 in 1986, using a hollow spoke wheel system developed by PSK. In 1999 the PSA Peugeot Citroen decided to adopt TPM as a standard feature on the Peugeot 607. The following year (2000), Renault launched the Laguna II, the first high volume mid-size passenger vehicle in the world to be equipped with TPM as a standard feature.

In the United States, the Firestone recall in the late 1990s (which was linked to more than 100 deaths from rollovers following tire tread-separation), pushed the Clinton administration to legislate the TREAD Act. The Act mandated the use of a suitable TPMS technology in all light motor vehicles (under 10,000 pounds), to help alert drivers to severe under-inflation events. This act affects all light motor vehicles sold after September 1, 2007. Phase-in started in October 2005 at 20%, and reached 100% for models produced after September 2007. In the U.S., as of 2008 and the EU, as of 2012, all new models of passenger cars must be equipped with a TPMS.

After the Tread Act was passed, many companies responded to the new market opportunity by releasing TPMS products that use an obvious means of getting tire pressure and temperature data across a vehicle’s rotating wheel-chassis boundary - battery powered radio transmitter wheel modules. Today, a plethora of different systems, each with product capabilities and options, are sold in transportation markets worldwide, providing tire pressure monitoring for all vehicle types, sizes and configurations.

TPMS support for OEM (factory) systems has resulted in a confusing patchwork of proprietary implementations in the automotive industry sector which is vast and includes vehicle manufacturers and assembly plants, OEM dealers, tire fitters, auto electricians and the vehicle owners and drivers. Information and training in diagnosing OEM TPMS faults, correctly repairing/replacing the multitude of different TPMS components and in the different TPMS “Learn tools” and activation and reset procedures has become complex.

Regardless of U.S. and EU legislation, the introduction by several tire manufacturers of run flat tires and space saver tires has required TPMS to be mandatory. When under-inflated or "flat" the run-flats are designed to be used at no more than 80 km/h (50 mph) for no more than a distance of 80 km. The requirement is similar for space saver tires. Furthermore, transportation communities are realizing that visual inspections and “tire kicking” are not adequate means of testing, resulting in commercial vehicles utilizing TPMS, although not required by law.

In recent years, several advancements have been made in the TPMS market, especially in the commercial vehicle and aftermarket segments. New developments include battery-less systems, using electromagnetic coupling technology to power sensors.

Direct vs Indirect

Direct TPMSs employ pressure sensors, either internal or external, on each tire which physically measure the tire pressure in each tire and report that information to the vehicle's instrument cluster or a corresponding monitor. These systems can identify under-inflation situations, simultaneous or singly, in any combination. Though systems vary in reporting options, many TPMS products (especially aftermarket solutions) can display real time tire pressures at each location monitored whether the vehicle is moving or parked.

Direct-sensor TPMS are specifically designed to cope with ambient and road-to-tire friction-based temperature changes, both of which heat up the tire, and increase its pressure. The alarm-activation threshold pressure for OEM systems is usually set according to the manufacturer's recommended "cold placard inflation pressures". Aftermarket system thresholds, which vary per system, can either be set at the factory or can be set by the customer during installation.

Direct sensors require power and may be either battery-powered or battery-less. In order to transfer data to the monitor or display, most direct systems utilize sensors having battery powered radio-frequency (RF) communication to transmit pressure readings and other data collected from the sensors.

Generally, the pressure sensors used in direct-sensor TPMS can be installed inside the tire (internal sensors) or mounted on the valve stem (external sensor). Internal sensors have to be fitted to the wheel rim or to the in-tire section of the valve stem when the tire is fitted, requiring tires to be dismantled when installing or maintaining sensors. Additional interference from the metal of a tire typically requires that TPMS systems utilizing internal sensors use additional antennae to get signals to the cab on larger vehicle configurations. In comparison, external sensors are installed by being screwed onto the tire’s valve stem, replacing the dust cap, but are vulnerable to theft and physical damage.
  • Typical system
    In most current designs of direct TPMS, a small electronic assembly which is rugged enough to be mounted inside a tire, measures the pressure using a microelectromechanical system (MEMS) pressure sensor and then transmits this and other information to one or more vehicle receivers. Other information can include a serial number, temperature, acceleration and the status of the complete tire pressure monitoring system. The purpose of the serial number is to allow the vehicle to ignore transmissions from other vehicles and operate with a unique data field. A typical direct TPMS (e.g. Ford, BMW or Toyota) comprises the following components on a vehicle:
    • A direct TPM sensor fitted to the back of the valve stem on each wheel
    • A TPM Warning Light
    • Unique identifier (ID's) for which tire is providing the data including speed and the direction of rotation
    • A tire pressure monitor electronic control unit (ECU)
    • Antenna(s)
    • Controller for periodic measurements
    • Source of power
    • Diagnostics and wake up system

    Most direct TPMS systems use ultra high frequency (UHF) radio in one of the 'unlicensed' ISM bands (industrial, scientific and medical) for transmitting the data, often around 434 MHz in Europe and 315 MHz in much of the rest of the world. On some systems there is a separate receiver or antenna near each wheel whilst more commonly there is a single receiver which receives data from all of the wheels on the vehicle. Commonly this receiver is also used for remote keyless entry system (RKE) as this also usually uses UHF radio transmissions.

    TPM sensors can be fitted to the wheels in a number of ways. They can be mounted on the back of the tire valves stem or attached using adhesive or to a band which is then securely wrapped around the rim inside the tire, usually in the drop zone.

  • Direct tire pressure monitor system warning light
    When the direct TPMS warning light comes on, either one of the tires is under-inflated or there is a fault with the system. If the light is constant then inflating to the correct placard pressure should turn it off. If this is not the case then this indicates a puncture. If the light is intermittent or if it stays on after correct inflation or replacement of a punctured tire then this indicates a fault with the direct TPMS system.

  • Registration of direct TPMS ID's
    When the direct TPMS system is fitted at the factory the unique ID numbers of the TPM (tire pressure monitoring) sensors have to be registered along with their position on the vehicle with the tire pressure monitor ECU. This is also the case if any of the system components are subsequently changed e.g. in the event of rotating the tires, changing sensors, replacing the ECU etc. This process requires the activation of the direct TPMS sensor using low frequency (LF) radio and the capture of the UHF data transmitted. This data includes the direct TPMS ID, the pressure and the temperature. In automotive manufacturing plants, the activation is carried out using large antenna systems whilst in the dealerships and tire shops, hand tools are used. These tools can also be used to check the direct TPMS for faults prior to disassembly. If a TPM sensor or its position on the car are changed without re-registering the ID's, then the TPMS warning light will turn on and stay on until the ID's are re-registered.

  • Localisation
    If there are multiple antennas or receivers, this permits localisation of the TPM such that the vehicle can tell from which wheel the pressure data has come. As an alternative to this method, the vehicle can be programmed at the time of manufacture with the position of the tire together with its TPM serial number. This allows the vehicle to display which tire has low pressure.

    Also, some vehicles have low frequency radio transmitters mounted near to each wheel which can be used to force the individual TPMs to transmit at will. These typically use similar technology to 125 kHz RFID tags where the transmitted field is predominantly magnetic and can be easily detected by a small LF antenna located in the TPM. This method of localisation is often referred to as a high line system.

    The LF antenna is also often used by the TPM for configuration and to force transmission so that localisation can be re-learned by the vehicle if a sensor is changed or the wheels rotated to even up tread wear.

    A third method uses the UHF signal strength which is proportional to the distance of the TPM from the receiver. If the receiver is located towards the front of the vehicle, the signal from the front wheel TPM's will be stronger than that from the wheels at the rear.

    The TPM also has a method of detecting the rotational direction of each wheel to identify which side of the vehicle the TPM is located and this information forms part of the message transmitted to the vehicle. This combination allows the correct wheel to be identified.

  • TPM sensor features
    The TPM sensors currently fitted to high volume production cars worldwide are battery-powered, self-contained units which periodically measure tire pressure, and often temperature and acceleration. The sensor is equipped with an RF transmitter circuit which is used to broadcast the measured pressure etc. within the tire.

    The TPM is designed to use as little power as possible to give maximum battery life. This is done by using very low power circuitry and transmitting the data as infrequently as possible and with as low power as possible. The UHF transmitter in a TPM typically transmits around 250?W (1?W is equal to one millionth (10?6) of a watt).

    TPM's do not have UHF receivers built in due to the relatively high power requirements of this technology. This means that they can't tell that they are transmitting at the same time as another TPM. Most TPM's do have LF receivers as this uses little or no power.

    The pressure, temperature and acceleration sensors generate analog signals which are converted to their digital equivalents using analog to digital converters. The acceleration sensor measures the centrifugal force generated when the wheel rotates. This force is proportional to the rotational speed. The acceleration sensor may be a simple switch rather than an analog transducer (accelerometer). This is usually referred to as a roll switch. The acceleration sensor allows the TPM to be placed in a low-power communication device mode, when the vehicle is stationary which can extend the battery life. The advantage of a roll switch over an accelerometer is that the switch is purely mechanical and doesn't use any power to take a measurement.

    When the vehicle is stationary, the TPM may periodically transmit to the vehicle. This allows (as long as the vehicle receiver is always on) the driver or vehicle operator to be warned of low pressure as soon as the Ignition system is switched on rather than having to wait until the vehicle is moving.

    All TPM units on a vehicle operate on the same RF channel frequency and each message includes pressure data, temperature data, a unique ID code, operating state data, status information and check digits. The check digit is either a checksum or a cyclic redundancy check (CRC).

    The TPM does not usually have information about the tire's correct pressure as this would be very difficult and possibly dangerous to support. However it may have an algorithm contained within it which detects both slow and rapid changes in pressure. This condition may be transmitted as part of the TPM's status. It may also cause the TPM to transmit more frequently.

  • High line system
    If the vehicle is fitted with low frequency (LF) transmitters near each wheel, the vehicle may use these to force the sensors to transmit. In this case, the TPM may not transmit on its own, but the vehicle will periodically command the sensors to send their information.

    In addition, the TPM's will be forced to transmit when the ignition is switched on. This will give an early indication of low pressure without having to have the vehicle's receiver switched on when the vehicle is not in use. The transmitters are usually activated one at a time in sequence so that the vehicle can inform the driver of the location of the wheel with low pressure. This information can then be used later for localisation by matching the TPM's unique ID with its position discovered by this sequential activation. This method is used on some high line systems where the TPM also transmits periodically.

    On some vehicles only three LF transmitters are used in order to save money. The vehicle assumes that transmissions from a nearby TPM which has not been woken up by the LF belong to the TPM located where there is no LF transmitter.

    High line systems are inherently more expensive than low line systems but they have the advantage of the vehicle knowing the pressure when started without draining the vehicle's main battery and providing localisation. These systems tend to be used on higher end models.

  • Low line system
    In this system, the TPM units transmit on their own at fixed or random intervals. As the individual TPM's on the vehicle do not know if another TPM is transmitting at the same time, it is possible to have collisions between messages transmitted. Measures have to be taken to ensure that the message is received by the vehicle. On some systems the message is re-transmitted multiple times to reduce the effect of interference (communication). The transmission pattern can be random or pseudo random to reduce the chance of collisions between transmissions from the sensors on the vehicle.

    Another method of attempting to avoid collisions is simply to transmit more frequently such as once per minute. In addition, if the TPM detects a rapid change in pressure or too high a temperature, it will start to transmit more frequently so that the vehicle has more chance of receiving the information. The low line system is used on the majority of vehicles due to its lower cost.

Indirect TPMS do not use physical pressure sensors but "infer" air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself. Most indirect TPMS systems operate under the theory of using an under-inflated tire’s slightly smaller diameter (and hence higher angular velocity) to determine inflation. Later developments of indirect TPMS can also detect simultaneous under-inflation in up to all four tires using vibration analysis of individual wheels or analysis of load shift effects during acceleration and/or cornering, which can be realized in software using advanced signal processing techniques. The vibration analysis technique may require the use of additional suspension sensors, which result in increased complexity and cost of the overall system, as long as vertical chassis movements are concerned. That is why most current advanced indirect systems use the spectral content of the wheel speed sensor signals, so no additional sensors are needed. The computations can also be carried out by existing computing power, for example in usual ABS or ESC control units.

Indirect TPMS are realized in software algorithms in combination with wheel-speed sensors for anti-lock braking systems, and electronic stability control systems. An advantage of the indirect TPMS is that it needs no sensors, thus decreasing weight and cost as well as increasing customer satisfaction, because sensor-related problems are eliminated. A disadvantage of indirect TPMS is that the driver must calibrate the system by pushing a reset button on the dashboard via an on-board computer, and if this is performed when any tire is in an under inflated condition, then the system will report erroneously. Audi was the first car maker to attempt to comply with the U.S. TPMS legislation (Tread Act), using an indirect system, with the launch of the Audi TT model year 2006. The system - called Tire Pressure Indicator TPI - purports to meet the American FMVSS 138 and the European ECE R-64 safety regulations on tire pressure monitoring systems. Unfortunately, it does not yet comply with the US DOT NHTSA regulations.

Presently indirect TPMS systems do not meet the requirements of the US government NHTSA DOT as a viable solution for TPMS Tread Act requirements under FMVSS 138.

Benefits of TPMS
TPMS systems are designed to provide drivers with the tire pressure information and alerts needed to add safety and savings to travel through increased fuel efficiency, extended tire life, decreased downtime and maintenance, improved stability and handling, and decreased emissions. These are significant advantages and are summarized as follows:

  • Fuel savings: According to the GITI, for every 10% of under-inflation on each tire on a vehicle, a 1% reduction in fuel economy will occur. In the USA alone, the Department of Transportation estimates that under inflated tires waste 2 billion US gallons (7,600,000 m3) of fuel each year.
  • Extended Tire Life: Under inflated tires are the #1 cause of tire failure and contribute to tire disintegration, heat buildup, ply separation and sidewall/casing breakdowns. Further, a difference of 10 lbs. in pressure on a set of duals literally drags the lower pressured tire 13 feet per mile. Moreover, running a tire, even briefly on inadequate pressure, breaks down the casing and prevents the ability to retread.
  • Decreased Downtime & Maintenance: Under-inflated tires lead to costly hours of downtime and maintenance.
  • Added Safety: Under-inflated tires lead to tread separation and tire failure, resulting in 40,000 accidents, 33,000 injuries and over 650 deaths per year. Further, tires properly inflated add greater stability, handling and braking efficiencies and provide greater safety for the driver, the vehicle, the loads and others on the road.
  • Drive Green: Under-inflated tires, as estimated by the Department of Transportation, release over 57.5 billion pounds of unnecessary carbon-monoxide pollutants into the atmosphere each year in the US alone.

Further statistics include:
  • The French Securite Routiere, a road safety organization, estimates that 9% of all road accidents involving fatalities are attributable to tire under-inflation, and the German DEKRA, a product safety organization, estimated that 41% of accidents with physical injuries are linked to tire problems.
  • On the maintenance side, it is important to realize that fuel efficiency and tire wear are severely affected by under-inflation. In the U.S., NHTSA data relate that tires leak air naturally, and over a year, a typical new tire can lose from 20 to 60 kPa (3 to 9 psi), roughly 10% or more of its initial pressure.
  • The European Union reports that an average under-inflation of 40 kPa produces an increase of fuel consumption of 2% and a decrease of tire life of 25%. The EU concludes that tire under-inflation today is responsible for over 20 million liters of unnecessarily-burned fuel, dumping over 2 million tonnes of CO2 into the atmosphere, and for 200 million tires being prematurely wasted worldwide.

In the U.S., the U.S. Department Of Transportation (NHTSA) released the FMVSS No. 138, which requires an installation of a Tire Pressure Monitoring System to all new passenger cars, multipurpose passenger vehicles, trucks, and buses that have a gross vehicle weight rating (GVWR) of 4,536 kg (10,000 lbs.) or less, except those vehicles with dual wheels on an axle, as of 2007. In the EU, starting in 2012, all new models of passenger cars must be equipped with a TPMS, with even tighter specifications that will be defined by the UNECE Vehicle Regulations (Regulation No. 64). On July 13, 2010, the South Korean Ministry of Land, Transport and Maritime Affairs announced a pending partial-revision to the Korea Motor Vehicle Safety Standards (KMVSS), specifying that "TPMS shall be installed to passenger vehicles and vehicles of GVW 3.5 tons or less, ... [effective] on 1 January 2013 for new models and on 30 June 2014 for existing models".Japan is expected to adopt EU legislation approximately one year after EU rollout.

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